Effect of serotonin on delay activity in an RS cell. A, In the control condition the cell shows barely any distinction in its response for the 0° and 180° targets. However, on application of 5-HT at just 10 nA there is a marked enhancement of the firing of the cell that particularly accentuates the delay activity on 0° trials. Subsequent coapplication of MDL100,907 (bottom panel) dramatically reduces the firing rate and attenuates the previous selective response in the delay period. B, The delay activity of this RS cell does not show any spatial specificity in control but it develops into a significant memory field (TF = 1) when 5-HT is applied (middle panel). Coapplication of MDL100,907 produces a substantial reduction in the delay activity but sharply limits firing to a small region of space and, as a consequence, improves spatial tuning (TF = 2). Conventions as in previous figures.